This invention relates to electrically programmable memory (EPROM) cells, and particularly to a new arrangement for writing and reading data into and from such cells.
EPROM cells comprise a metal-oxide-semiconductor (MOS) device including a control gate electrode overlying a channel region of the device and an ohmically isolated floating gate electrode disposed between the control gate electrode and the channel region. Because of the ohmic isolation of the floating gate electrode, electrons reaching it remain stored thereon and provide a voltage bias which affects the turn on characteristics of the MOS device.
For example, with an NMOS device (N type source and drain regions separated by a P type channel region) having a sufficient quantity of electrons stored on the floating gate, the device is biased in its off state, and requires the application of a positive voltage to the control gate electrode to turn it on.
The presence or absence of electrons stored on the floating gate electrode corresponds to either the storage of a logic 1 or 0 in memory. Reading of the memory comprises applying a voltage to the control gate electrode for changing, or not changing, the on-off state of the MOS device depending upon the presence or absence of the stored electrons.
The writing of data into such cells comprises the collection of electrons onto the floating gate electrode through the insulating material disposed between it and the channel region. This is done by applying a relatively large positive voltage to the control gate electrode and the drain (with respect to the source) which generates a stream of high energy "hot electrons" through the channel region. Owing to the high positive voltage on the control gate electrode, which is capacitively coupled to the floating gate electrode, some of the high energy channel electrons have sufficient energy to overcome the insulating barrier layer, and are attracted to the floating gate electrode. When the data writing voltages are turned off, the electrons which have reached the floating gate electrode remain trapped thereon.
During subsequent reading of the cell, the voltages used to determine the on-off state of the MOS device are sufficiently low to prevent further tunneling of electrons to or from the floating gate electrode, thereby providing a relatively permanent storage of the written data.
One condition necessary for inducing the tunneling mechanism is that the channel region of the MOS devices be relatively short, e.g., approximately one micron or shorter in length, in order that sufficiently high electric fields are present to generate the necessary high energy electrons. MOS devices having such short channels are available, but require high resolution photolithographic manufacturing processes. Thus, in semiconductor devices which are conventionally manufactured with lower resolution processes, it has not been possible to use EPROM cells of the type described herein.
This limitation is overcome in accordance with this invention.